The present invention relates generally to apparatus useful in locking or holding down one member within another member, and/or in actuating a seal member or other mechanism associated with the members. The present invention is particularly useful in connection with the production of oil and gas, in the form of a lockdown screw which may be used in a wellhead housing, a casing or tubing head, a spool, or another such member to hold down a casing hanger, tubing hanger, or other apparatus therewithin, to actuate a seal member between the hanger or other apparatus and the member within which it is disposed, or for other purposes.
In the production of oil and gas, it is common practice to use lockdown screws in wellhead housings, casing or tubing heads, spools, or the like to hold down casing or tubing hangers or other equipment therewithin. Such lockdown screws are also commonly used to energize or actuate seal members or other mechanisms disposed between the hangers or other equipment and the housings or heads for establishing a fluid-tight seal between the members, or for accomplishing some other purpose. Typically, lockdown screws used in the past have included a gland nut threaded into a first bore in the housing or head, a stem threaded into a second, reduced diameter bore in the housing or head, the second bore typically being coaxial with the first bore, for longitudinal movement into or out of the housing or head upon clockwise or counterclockwise rotation, as the case may be, of the stem in the second bore, and a packing between the stem and the housing or head, actuated by tightening the gland nut, for sealing between the stem and the housing or head. Thus, lockdown screws used in the past have typically had two threads in the housing or head, the first being that in which the gland nut operates and the second being that in which the stem operates. The latter thread is typically referred to as the working thread. Both threads are usually placed in a flange or the like in the housing or head. For large flanges, the threads of the second bore are thus disposed at relatively large distances from their outside surfaces.
From time to time, leaks may develop in the stem packing, permitting the escape of fluids from the housing or head at the location of the leaking lockdown screw. Such leaks may be extremely dangerous for an oil or gas well containing highly flammable and volatile petroleum products, which are usually under high pressure, possibly resulting in a fire, explosion, or other hazardous situation. In an extreme case, the eventual result could even be a blowout.
Sometimes a lockdown screw leak can be corrected by simply tightening the gland nut, increasing the energizing force on the packing material and forcing the packing material more tightly against the housing and the stem. At other times, however, such a procedure may not be sufficient to stop the leak, or may not be appropriate even if the leak were to be stopped. For example, the gland nut may already be tightened to an extent such that it cannot easily be tightened further, or if tightened further it might cause damage to the threads on the gland nut or in its threaded bore. Damage to the threads in the first bore or on the gland nut might in turn exacerbate the leak. Thread damage in the first bore may require costly and time-consuming repair, as discussed further below. Moreover, in a situation where one or several lockdown screws of a greater plurality of such screws is or are tightened to varying degrees in order to stop leaks, the load applied by such screws to a compression packoff or the like becomes uneven or uncertain, possibly resulting in improper packoff performance, or even damage to the packoff.
Thread damage in the second threaded bore also occurs from time to time and is particularly troublesome, since in most cases the threads in the second bore will extend from the end of the first threaded bore along the entire remaining length of the bore, deep into the housing or head as referred to above, and it is not always easy or convenient to repair damaged threads of this type. Such repair usually requires plugging, welding, and reboring the damaged bore, or boring an entirely new threaded bore adjacent to the plugged bore. The same can be said of damage to the threads in the first bore. It may be difficult to make such repairs in the field; a trip to the shop may be required. Damage to the working threads, i.e., those in the stem bore, can occur, for example, if the stem were to be cross-threaded or overtorqued, if the stem were to be dropped or otherwise mishandled, or for other reasons. Moreover, since the working threads are usually exposed to mud, oil, dirt and other contaminants from well fluids, because the stem packing is typically disposed radially outwardly of the working threads, the risk of damage to such threads is greatly enhanced.
If the operators are unable to stop the leaking lockdown screw by simply tightening the gland nut, usually the well will have to be shut in and production or other operations stopped, or a special packer will have to be run into the well to isolate the wellbore from the leaking lockdown screw, in order to correct the problem. It may be that simply replacing the packing or the entire lockdown screw will stop the leak, but if there is any significant damage to the threads in the stem bore or the gland nut bore, then repairs of the type referred to above may be necessary. In addition, frequently the stem packings become stuck in their bores, making it difficult to remove them and unduly complicating the packing replacement operation. Downtime for high-priced rig crews and equipment while waiting for repairs to be made can be prohibitively expensive, especially if off-site repair is required.
In addition to the problems discussed above, there is a serious problem with prior art lockdown screw designs that allows operator error to create potentially hazardous, even catastrophic, conditions at the well site. In the past, in order to run a lockdown screw when desired or required by well operations (typically when the hanger is landed or otherwise packed off with the lockdown screw), frequently the gland nut is or must be loosened to allow the lockdown screw stem to be rotated easily. This in turn de-energizes the packing around the lockdown screw stem. If the operator then forgets to re-tighten the gland nut when the lockdown screw is placed in service and the well is opened or the packoff isolating the lockdown screw removed, a leak is likely to occur at the subject gland nut, possibly leading to a fire, explosion, or blowout as discussed above. Since there are typically many such lockdown screws to be alternately placed into and taken out of service in a conventional wellhead installation, the probability of an operator's forgetting or neglecting to re-tighten all of the gland nuts once they have been loosened is substantial.
It is an object of the present invention to provide a lockdown screw for use with a wellhead housing, casing head or tubing head, spool, or other such member which overcomes the problems of prior art lockdown screws referred to above. It is another object of the present invention to provide such a lockdown screw which has a self-energized packing which does not require the gland nut to be screwed tightly into its receiving bore in order to actuate the packing, minimizing the possibility of damage to the threads in the bore and on the gland nut. It is yet another object of the present invention to provide such a lockdown screw with a self-energized packing wherein the working thread is adapted for stem actuation only and is placed in the gland nut instead of the flange of the housing or head, resulting in both easier manufacture of the housing or head and easier repairs to damaged or leaking lockdown screws in the event such repairs are necessary. It is also an object of the present invention to provide such a lockdown screw which has a metal-to-metal backseat seal which may be quickly and easily actuated in the event that a leak develops around the stem.
It is also an object of the present invention to provide such a lockdown screw having a stem which may be easily actuated both into and out of the housing or head during service without the need to back off the gland nut first, thereby eliminating the possibility that an operator will forget to retighten a gland nut after loosening it to actuate the lockdown screw stem.
It is a further object of the present invention to provide effective metal-to-metal seals between the gland nut and the stem and between the gland nut and the housing or head which may be actuated in the event of a leak developing at the lockdown screw during service, providing a positive, reliable and secure means for stopping the leak without shutting in the well or otherwise interrupting service. It is an additional object of the present invention to provide such a lockdown screw with a metal-to-metal seal around the stem between the stem and the nose of the gland nut, and a metal-to-metal seal around the exterior of the gland nut between the gland nut and the housing or head, which may be quickly and easily actuated in the event that a leak in the lockdown screw is not stopped by actuation of the metal-to-metal backseat seal.
It is a further object of the present invention to provide a lockdown screw wherein the sole working thread is isolated by the packing from well fluids, eliminating the risk of damage to the working thread from such well fluids. It is an additional object of the present invention to provide means for easy removal of the packing from the housing or head when the lockdown screw is removed. It is another object of the present invention to provide such a lockdown screw which is relatively simple, easy to manufacture and install, inexpensive, and reliable.